The field of this invention is generally related to tachometers for indicating engine speed, and is more specifically related to tachometer systems which emit an audible signal when a desired engine speed is attained.
In auto racing, drag racing, or other vehicle operation, wherein the attainment of maximum speed or optimum efficiency is desired, the transmission of the vehicle must be shifted at specific engine speeds. A driver of such a vehicle must monitor a tachometer to insure that he shifts the gears at optimum engine speeds. If the tachometer provides only visual indications of speed, the driver is distracted from observing the roadway in order to monitor the tachometer. Thus, for both safe and efficient operation of such vehicle, it is therefore desirable that the driver be given a signal indicating the proper time to shift gears, where thereby avoids distracting him from observing the roadway. An audible signal is ideally suited for that purpose.
A number of tachometer systems have been devised in the prior art for emitting an audible "alert" signal, when an engine has attained a specific speed. For example, Magid, U.S. Pat. No. 2,728,072, discloses a speedometer having an uninsulated speedometer needle 4 electrically grounded through a hair spring 8, the needle being movable to indicate vehicle speed upon a dial graduated in miles per hour. A contact member 18 is rotatably mounted on a shaft 16, wherein the contact 18 also serves as a pointer to permit it to be manually set at a desired speed. A free end of the pointer like contact member 18 is bent ninety degrees from its longitudinal axis, the free end being bent into the path of travel of the needle 4. When the needle 4 moves to the vehicle speed graduation at which contact 18 has been set, an edge portion of the needle 4 will engage the bent end of the contact member 18 to provide a ground path for an external circuit. Upon engagement of the needle 4 and contact member 18, current will flow from the external circuit, through the contact member 18, the needle 4, a shaft 6 to which the needle is attached, a hairspring 8 also attached to the shaft 6, and from the hairspring to ground. The external circuit includes an electrical buzzer 26, which is energized whenever needle 4 engages contact member 18, whereupon an audible buzz is emitted from the buzzer 26 to alert a driver that the vehicle has attained the speed at which contact member 18 is set.
The Magid patent also discloses a tachometer, as shown in FIG. 6, which is similar in construction to his speedometer, but includes two manually settable contact members 62 and 66 for engaging a needle 68 at different times to complete a ground path for an external circuit whenever the engine speed drops to a magnitude equal to that indicated by contact member 62, or increases in speed to a magnitude equal to or greater than that indicated by contact member 66. The Magid patent further discloses the use of a microswitch as his contact means, or the use of photoelectric means, in cooperation with a meter needle.
Baron, U.S. Pat. No. 2,963,694, discloses a speed alarm for vehicles including a cable driven speedometer having a manually settable pointer 32 mounted upon a mounting block 30. The mounting block 30 is rotatably mounted upon a shaft 24. A cantilever-like contact carrying member 36 has an end rigidly attached to the mounting block 30. The free end of the contact carrying member 36 has a button contact 38 rigidly mounted thereto. The speedometer also has a speed indicating needle 46 mounted upon another mounting block 44, the mounting block 44 being rigidly affixed to shaft 24. Also, another contact carrying member 42 is rigidly attached at one end to block 44 and has a button contact 40 attached to its free end. The contact carrying members 36 and 42 are aligned so that when their respective manually settable pointer 32 and needle 46 are indicative of identical vehicle speeds, their respective contacts 40 and 38 are closed upon one another thereby completing a series electrical circuit including a bell, whereupon the bell is energized to provide an audible signal to the vehicle driver indicating that a certain vehicular speed has been attained.
Prior art audio tachometer systems are characterized by a number of drawbacks, depending upon the system design. For instance, such systems utilizing cable and gear driven tachometers tend to be inaccurate due to backlash in the gearing and cable.
Systems using tachometers having driven needle or pointer speed indicators, which incorporate an electrical contact assembly with the driven needle must supply an inordinate magnitude of rotational force to the needle speed indicator and contact assembly to move the composite assembly to the dial graduation which indicates the actual engine speed. Such systems suffer from poor response time to abrupt changes in the speed of the engine, decreased reliability due to the extra wear upon moving parts, and increased assembly costs. Reliabilty is also decreased by the continual flexing of the wire connected to the electrical contact assembly that must move back and forth as the speed indicating needle moves in accordance with the changes in the engine speed.